P0420 — Catalyst efficiency below threshold (Bank 1)
Your car’s computer thinks the catalytic converter on bank 1 isn’t cleaning exhaust as well as it should.
What it means in plain English
P0420 means the engine computer compared upstream and downstream oxygen-sensor readings and decided the catalyst on bank 1 (usually the side with cylinder 1) is below its efficiency threshold. It is a monitored condition, not a parts order. A failing converter can set it — so can a lazy O2 sensor, an exhaust leak before the downstream sensor, wiring issues, or an engine that has been running rich/lean or misfiring.
Common causes
- Worn or contaminated catalytic converter
- Faulty or slow upstream/downstream oxygen (O2 / A/F) sensor
- Exhaust leak ahead of the downstream sensor
- Ongoing misfire, rich/lean condition, or oil/coolant contamination damaging the cat
- Wiring or connector problems at the O2 sensors
What you can safely check
- Read and photograph all stored and pending codes — companion codes (misfire, fuel trim, O2) matter.
- Note whether the light is steady or flashing. Flashing usually means active misfire — ease off and get help.
- Check for obvious exhaust leaks (loud ticking near the manifold) and recent fuel/oil issues.
- Do not clear the code before the shop sees freeze-frame data unless they ask after a verified repair.
Pull the code yourself (disclosed)
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A reusable Bluetooth OBD-II scanner lets you read the code and freeze-frame data before authorizing “mystery” diagnostics. One option: BlueDriver Bluetooth Pro OBD-II scan tool (BlueDriver/Levanta affiliate). Any capable OBD-II reader is fine — this is not required to use Fair Auto Shops.
When to go to a shop
Go to a shop when the light returns after a drive cycle, you smell sulfur/rotten eggs, power is down, or you need an emissions inspection. Ask the shop to confirm catalyst efficiency with live data (sensor switching comparison) before authorizing a converter. A converter-only estimate with no sensor/exhaust tests explained is a red flag — see spotting upselling.
Typical related repair costs (U.S. national averages)
These are related repairs shops may find after diagnosis — not a guaranteed bill for every P0420. Ranges exclude taxes, fees, diagnostics, and your location.
- $440–$523 national average for oxygen sensor replacement (RepairPal; estimates last updated September 3, 2026; excludes taxes/fees and location; source). Labor $93–$136; parts $347–$387.
- $2,280–$2,480 national average for catalytic converter replacement (RepairPal; estimates last updated September 3, 2026; excludes taxes/fees and location; source). Labor $236–$347; parts $2,044–$2,133.
Many P0420 cases are not an automatic new converter. If diagnosis points to a sensor or leak, expect the lower repair path first.
Related cost guides: Catalytic Converter · Oxygen Sensor
Questions to ask before you authorize work
Use questions before you authorize a repair and how to read a repair estimate. Watch for patterns in spotting upselling.
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FAQ
Does P0420 always mean a new catalytic converter?
No. Shops should verify with live O2 data and rule out sensors, exhaust leaks, and engine problems that poison the catalyst before quoting a converter.
Can I drive with P0420?
A steady light with normal drivability is usually not an emergency, but you can fail emissions and a failing cat can worsen. A flashing light is urgent.
More check-engine codes
P0430 · P0300 · P0301 · P0171 · P0174 · P0455 · P0456 · P0442 · P0446 · P0128 · P0401 · P0133 · P0113 · P0506 · P0507 · P0700 · P0741 · P0011 · P0325 · P0340 · P0335 · P0101 · P0299 · P0135 · P0440 · P0441 · P0010 · P0500 · P0121
Hub: How to read a check-engine code · Repair cost guides · Affiliate disclosure